1. Field of the Invention
The present invention relates to polyurethanes. More specifically, it relates to coating compositions containing polyurethane crosslinkers, and to metal containers coated with such coating compositions.
2. Brief Description of the Prior Art
Generally, crosslinkers which are used in coatings would have an effect on the performance characteristics of said coatings. In can coatings, which are of a particular interest, the crosslinkers which are used therein must be such as would enable the can coating to meet very special (stringent) requirements. Such coatings must have excellent chemical properties, excellent fabricating properties, i.e., a combination of flexibility, extensibility and adhesion, so as to permit forming operations to be carried out on the coated metal without cracking or otherwise impairing the continuity of the film; excellent intercoat adhesion (e.g., adhesion to topcoat material); excellent adhesion to the bonding adhesive employed on the lap seam; and resistance to the high temperatures employed in container production or during pasteurization of the beverage (e.g., beer). Yet another requirement: even though the size coat (exterior) and base coat (interior) may not be in direct contact with the packaged beverage, the possibility exists that due to incomplete topcoat coverage or accidental exposure in the seam area; these coatings should, in this regard, be formulated so that they do not adversely affect the taste characteristics of or cause turbidity in the packaged beverage.
As will be recognized from the above, the production of container coating materials meeting all of these requirements is a difficult undertaking. Furthermore, the problem of producing successful coating materials of this type has been even further complicated by the increasing national emphasis on ecological and environmental concerns.
It is believed that the crosslinkers, employed in the preparation of the coatings, would have significant effects on coating properties of chemical resistance, flexibility and adhesion. To ensure these properties, much work has been done in the prior art relative to the crosslinkers. The challenge, in this regard, has been in providing coatings having, concurrently, the properties of chemical resistance, flexibility and adhesion. Meeting the desired standard of one of these properties (i.e., flexibility, adhesion or chemical resistance) had generally been at the expense of the other or others.
The present invention provides polyurethane crosslinkers, particularly water-reducible polyurethane crosslinkers, the coatings of which have been found to possess, concurrently, excellent properties of chemical resistance, flexibility and adhesion.